This invention relates generally to communication systems, and more specifically to communication systems having two-way communication capability.
Communication systems, such as cable television systems, typically include a headend section for receiving satellite signals and demodulating the signals to an intermediate frequency (IF) or baseband. The down converted signals are then modulated with radio frequency (RF) carriers and converted to an optical signal for transmission from the headend section over fiber optic cable. Optical transmitters are distributed throughout the cable system, such as at headends or hubs, for transmitting and/or forwarding optical signals, and optical receivers are provided in remote locations within the distribution system for receiving the optical signals and converting them to radio frequency (RF) signals that are further transmitted along branches of the system over coaxial cable rather than fiber optic cable. Taps are situated along the coaxial cable to tap off downstream (also referred to as xe2x80x9coutboundxe2x80x9d or xe2x80x9cforwardxe2x80x9d) cable signals to subscribers of the system.
Communications as described in the preceding paragraph are generally referred to as xe2x80x9cforwardxe2x80x9d or xe2x80x9cdownstreamxe2x80x9d communications since the signals originate at a headend and travel downstream, or in a forward direction, throughout the system to system subscribers. Some communication systems, particular some cable television systems, also include reverse path communications, in which subscriber equipment, e.g., set top boxes, televisions, and modems, transmit signals upstream, or in a reverse direction, to a headend or hub for processing. Communications in both directions have typically been analog in format.
Various factors influence the ability to accurately transmit and receive optical signals within an analog cable television system. As the length of fiber optic cable within a system increases, for example, signal losses also increase, thereby causing signal quality degradation. Furthermore, temperature fluctuations, which cause variation in the optical modulation index of the optical transmitter, can result in variation of the radio frequency (RF) output level of the optical receiver. Signal distortions can be caused by non-linearities in the laser and photodiode of the optical transmitter. These problems can be magnified when reverse path signals from subscriber equipment are transmitted upstream and processed by the same system equipment, such as nodes, hubs, and headend equipment.
Although signal degradation problems can be mitigated by employing expensive techniques, e.g., decreasing fiber lengths between optical nodes or increasing the number of hubs and nodes within a system, such techniques may prohibitively increase costs to both subscribers and service providers. Thus, what is needed is a better way to provide reliable and accurate transmission of optical signals within a cable television system.